This invention relates to an underwater detection system for searching the water within a wide angular range. Particularly it relates to a system which has a transmitting and receiving unit for transmitting an ultrasonic wave pulse into the water within a narrow sector of a predetermined angle and receiving echo signals by a plurality of reception beams, and turns the transmitting and receiving unit by a predetermined angle to search another narrow sector each time the search within the preceding narrow sector is completed, thereby searching the water within a wide angular range.
As one of prior art underwater detection systems, a PPI sonar has been widely used. A PPI sonar is disclosed in U.S. Pat. No. 2,759,783. The PPI sonar slowly rotates a transducer which radiates an ultrasonic wave pulse into the water in a direction and receives echo signals therefrom with a reception beam, thereby transmitting ultrasonic wave pulses in successive different directions and hence searching the water within a wide angular range. One of the drawbacks of the PPI sonar is its slow speed of searching. Assuming now that the beam width of a transmission beam and a reception beam is six degrees and the detection range of the sonar is 750 meters, it takes one second to search the water in a given direction and hence it takes sixty seconds to complete the search in all directions i.e., 360 degrees. Since the search in each direction is made every sixty seconds and the ship which is equipped with the PPI sonar advances, there will be areas where the search is not made.
Another prior art underwater detection system as disclosed in U.S. Pat. No. 4,045,766 has also been widely used. The prior art system has a plurality of transducers disposed on a circle and successively forms a series of directional reception beams along a circumferential direction at high speed, thereby searching the water in all directions very rapidly. A narrow reception beam is formed in a direction by appropriately phaseshifting the reception signals caught by each of selected groups of transducers. Reception beams will be successively formed in a circumferential direction by selecting different groups of transducers. But the construction of such a system will be complicated and the system inevitably will be bulky and costly.